This invention relates to logic systems in general and more particularly to a circuit arrangement for carrying out levels and requiring different supply voltage.
Various logic and switching systems have been developed, with each system using operating voltages which are selected primarily in dependence on the use to which the system is to be put. Thus, in logic systems for computers it is customary to use relatively low logic levels and low logic level voltage swings. On the other hand circuits developed for heavy industrial use where a large amount of interference is expected generally used higher logic levels and require a higher supply voltage. In such systems there is a larger signal excursion between the logic levels which are interpreted as a logical "0" and those interpreted as a logical "1". Using large signal excursions and larger voltages has as its primary advantage a higher signal to noise ratio. Furthermore, by designing logic circuits to operate at higher voltages there is less possibility of the circuits being destroyed by spurious voltages which may occur in the apparatus. Computer circuits, on the other hand, are generally not subject to such inteference and need not be designed to withstand destructive voltages. Thus, they operate at lower logic levels and in turn have a lower signal to noise ratio or signal to noise margin.
There is, however, an increasing use of computers and computer technology in industrial controls. As a result, computer systems must be interfaced with logic systems operating at higher levels. Typical of the systems now being used are those in which a central computer, typically a small computer utilizing large scale integration techniques is used to accept inputs from and provide outputs to logic systems associated with industrial controls. Naturally, the computer operates on low logic levels compatible with its integrated circuit design. On the other hand, the industrial control logic systems operate on a considerably higher voltage in view of the requirements for high signal to noise margin and the ability to withstand voltage spikes and the like. As a result, a need for means to interface the two systems arises. In other words, means to convert from one set of logic levels to the other in both directions are required. Typically, such is carried out through the use of suitable level converters such as those described in Electronic 1971, No. 4, pp 111 to 116, FIG. 21.
However, where separate level converters are used for each input and output the system cost and complexity is greatly increased. Furthermore, problems arise in achieving a balanced signal to noise margin between the logic levels of the different logic systems. In view of this, the need for an improved system with avoids these difficulties existing in the prior art becomes evident.